#pragma warning(disable:4996)
#include<stdio.h>
//斐波那契数列的循环计算
unsigned long long Fib(int n) 
{
	int first, second, third;
	first = second = third = 1;
	while (n > 2)
	{
		third = first + second;
		first = second;
		second = third;
		n--;
	}
	return (unsigned long long)third;
}
int main()
{
	int n;
	scanf("%d", &n);
	unsigned long long ret = Fib(n);
	printf("%llu", ret);
	return 0;
